1. Field of the Invention
The present invention relates to noise suppressing apparatuses, and more particularly, to a noise suppressing apparatus for suppressing high-frequency noise radiating through an interface cable or a power cable from a circuit board.
2. Description of the Related Art
Conventionally, an inductance element having a coil provided either on the surface of ferrite or inside of ferrite has been utilized for suppressing noise leaking through a signal cable or a power cable.
A ceramic ferrite, which is commonly used for this type of inductance element, complies with Snoeks' limit. According to Snoeks' limit, the permeability of a high-permeability ceramic ferrite tends to start decreasing in a comparatively low frequency band. For example, when ferrite having a relative permeability of 500 is used, there is a tendency for the permeability of the ferrite to start decreasing in the frequency band above several megahertz and to further decrease at higher frequency bands.
In the above-described inductance element including a magnetic body such as the ceramic ferrite, a noise suppressing effect on the order of tens to hundreds of megahertz can be obtained in the low frequency band. Conversely, in a high frequency band, as the permeability of the magnetic body decreases, the noise cannot be suppressed adequately because of a decrease in the noise suppressing effect.
When a ceramic ferrite having a low permeability is used, a constant permeability is maintained up to a relatively high frequency band. However, there is a problem in that a desired impedance of the magnetic body in the high frequency band is obtained, which causes the impedance thereof to decrease in the low frequency band. When a ceramic ferrite having a relative permeability of approximately 15 is used, the permeability starts decreasing in the frequency band above approximately 100 MHz, thereby complying with Snoeks' limit. There is a problem in that the noise suppressing effect can be obtained in the frequency band below 1 GHz, whereas it cannot be sufficiently obtained in the frequency band above 1 GHz.
When a ceramic ferrite having high impedance is desired, the number of turns of the coil must be increased. However, the increase in the number of turns of the coil causes stray capacitance to increase as well. In the frequency band above a particular frequency, the stray capacitance allows noise to pass because it functions as a capacitor. Thus, the noise suppressing effect is not achieved sufficiently and thus, cannot effectively suppress noise.
T-type filters and .pi.-type filters, obtained by combining an inductor and a capacitor, are known for suppressing noise. These filters show a remarkable noise suppressing effect, due to the combination of characteristics of the inductance and of the capacitance, up to a particular frequency. However, it is noted that the noise suppressing effect cannot be obtained sufficiently in the frequency band above the particular frequency because influences caused by residual inductance and stray capacitance prevent the noise suppressing effect from functioning properly in the high frequency band.
For example, in Japanese Unexamined Patent Publication No. 8-204486, there is disclosed a signal transmission element which defines a known noise suppressing apparatus.
FIG. 8 shows a perspective view of the signal transmission element defining the noise suppressing apparatus. In FIG. 8, a signal transmission wire 52 defines a coil by being wrapped around an insulated magnetic body 51 in the form of a ferrite rod which has a rod-like shape, and is embedded in a magnetic resin 53 obtained by combining a magnetic metal powder and a resin. A pair of external electrodes 54a and 54b are provided on the magnetic resin 53 allowing an electric current to flow through the signal transmission wire 52, and an electrode for ground connection or a ground electrode 55 is formed so as to substantially cover the entire surface of the magnetic resin 53 between the pair of external electrodes 54a and 54b.
The signal transmission element is constructed to maximize use of a magnetic loss generated by a ferromagnetic metal powder by effectively applying a high frequency magnetic field inside of the ferromagnetic metal particles in spite of the skin effect of the ferromagnetic metal. This construction enables this element to positively absorb the high-frequency signal components occurring in the high frequency domain.
However, in this noise suppressing apparatus defining the signal transmission element, although the noise suppressing effect is obtained sufficiently in the high frequency band above a gigahertz waveband, while it is not obtained in the low frequency band below a gigahertz waveband. The cutoff frequency of this noise suppressing apparatus is defined as the frequency at which the attenuation value of the transmission characteristic is about -3 dB. Since a noise suppressing effect with the attenuation value of only -10 dB or lower is obtained in the low frequency band below a gigahertz waveband, it is difficult to obtain the desired noise suppressing effect substantially in the low frequency band below a gigahertz waveband.